1. Field
The present invention relates generally to data communication, and more specifically to techniques for selectively combining multiple non-synchronous transmissions from multiple signal sources (e.g., base stations) in a wireless (e.g., CDMA) communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice and packet data services for a number of users. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access techniques. CDMA systems may provide certain advantages over other types of system, including increased system capacity. A CDMA system is typically designed to conform to one or more standards, such as IS-95, cdma2000, and W-CDMA standards, which are known in the art and incorporated herein by reference.
A CDMA system may be operated to support voice and data communication. During a communication session (e.g., a voice call), a terminal may be in active communication with one or more base stations that ate placed in an “active set” of the terminal. While in soft handoff (or softer handover), the terminal concurrently communicates with multiple base stations, which can provide diversity against deleterious path effects such as fading and multipath. While in active communication or idle, the terminal may also receive signals from the base stations in the active set and/or other base stations for other types of transmission such as, for example, pilots, pages, broadcast messages, and so on.
For some CDMA systems (e.g., a W-CDMA system), the base stations are not required to be operated synchronously. When operated asynchronously, from a terminal's perspective, the timing of the base stations is not aligned, and the reference time for each base station is different from that of other base stations. In this case, similarly formatted messages transmitted from multiple asynchronous base stations are likely to be received at different times at the terminal. Moreover, even if the base stations are operated synchronously, similarly formatted messages may be transmitted to the terminal from multiple base stations at different times (e.g., due to queuing delay, loadings, and so on). In this case, from the terminal's perspective, the transmissions are only “approximately synchronous” with respect to these messages.
If the terminal receives similarly formatted messages from multiple “non-synchronous” base stations, which include asynchronous and approximately synchronous base stations, then these messages need to be processed in the proper manner to correctly recover the underlying message. Since the similarly formatted messages are received at different times at the terminal, these messages cannot be combined at the symbol level in a straightforward manner, like the symbols for multiple signal instances (or multipaths), prior to the decoding to recover the message. If the underlying message cannot be recovered error-free from any single received transmission, then it may be necessary to combine multiple received transmissions in such a manner that the message can be recovered error-free.
There is therefore a need in the art for techniques to selectively combine multiple non-synchronous transmissions from multiple signal sources to correctly recover transmitted messages.